Known conventional permanent magnets include an alnico magnet as a casting magnet, a sintered ferrite magnet utilizing iron oxide, a rare earth metal magnet utilizing a transition metal, and the like. Among these magnets, a sintered ferrite magnet has been widely used in the field of household electric appliances, electronic equipment and heavy electric equipment.
However, in recent times electronic equipment require miniaturization and accuracy together with high performance and confidence. However, since the magnets prepared from a conventional permanent magnet have a simple shape and low dimensional stability, they cannot satisfy the above-mentioned requirements. Therefore, in order to overcome the limitations of permanent magnets and obtain complicated and various shapes and good dimensional stability, plastic magnets have been generally used.
Meanwhile, in the recent electric or practical electronic fields efforts to provide a flame-retardant property to plastic magnets as above have continued. Thus, when the plastic is contacted with flame, the so-called UL-94 standard which requires that flaming time and glowing time reach a certain level, should be satisfied.
Developments of such technique are revealed in some patents, for example, U.S. Pat. No. 4,490,498. In this patent, a polyolefin resin such as polypropylene powder was used as a binder resin, and a composition consisting of a halogen flame-retardant agent such as decabromodiphenyl ether, antimony trioxide, zinc borate hydrate and magnetic ferrite powder was utilized. The characteristic of this technique resides in improving a glowing combustion resistance by adding zinc borate hydrate.
In addition, in Japanese Patent Publication No. (sho)58-41643 a copolymer of hexamethylene adipamide and .epsilon.-caprolactam as a binder resin with ferrite was used. In the detailed description of the invention of this Japanese Patent it was disclosed that a halogen-base or nitrogen-base flame-retardant agent can provide a flame-retardant property.
However, the method of the above U.S. Patent is not suitable for use in electric or electronic products due to the intrinsic heat resistance limit of olefin resins including polypropylene and further to their insufficient mechanical strength. The technique of the Japanese Patent Publication the disadvantages that the dispersibility of magnetic powder is low and, therefore, deterioration of physical properties is serious, in addition to the disadvantages as mentioned above.
Accordingly, the present inventors have conducted extensive research to overcome the disadvantages of the prior techniques and thus have discovered a novel flame-retardant magnetic composite resin composition which has an improved heat resistance, does not show deterioration of physical properties by addition of a flame-retardant agent, and has significantly improved physical properties and processability due to good dispersibility of magnetic powder.